I argue that the knowledge argument is best understood as an argument for the existence of non-physical properties of material objects, or colours. I suggest that the knowledge argument is standardly presented as an argument for the existence of qualia because it is implicitly assumed that physics “tell us” that what it is like to perceive colour is determined, not by properties of material objects, but by properties of perceiving subjects; hence any gaps in Mary’s knowledge must be gaps in her knowledge about perceiving subjects. If nothing else, this physicalist assumption is odd given that the knowledge argument is supposed to be an argument against physicalism. Using the knowledge argument as an argument for the existence of non-physical colours is consistent with the transparency of perceptual experience. Moreover, rejecting the physicalist assumption behind the orthodox interpretation of the knowledge argument undermines the motivation for thinking of non-physical colours as epiphenomenal.